1. Field of the Invention
The present invention relates to a lithium-ion secondary battery.
2. Description of the Related Art
In recent years, reduction of carbon dioxide emission has been desired for environmental protection. In the automotive industry, introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been raising expectations for reduction of carbon dioxide emission. Developments have been made of secondary batteries for powering driving motor that are the key to practical application of these vehicles. In particular, a lithium-ion secondary battery is considered to be suitable for an electric vehicle application due to its high energy density and high durability against repeated charges and discharges. For this reason, developments of various lithium-ion batteries are now in progress.
As disclosed in Japanese Patent Application Laid-open Publication No. 2002-75455, a lithium-ion battery uses a lithium composite oxide and the like as an active material. A conventional active material is prone to deterioration under a high-temperature condition. For this reason, the deterioration of an active material progresses in the case where the temperature of the battery rises with heat generation by the passage of electric current. In response, there is a technique for suppressing such a rise in temperature by reducing a current to be supplied to a battery to some extent under a high temperature. Japanese Patent Publication No. 3811353 uses, as an electron-conductive material in contact with an active material, a material having a characteristic that electric resistance increases with a rise in temperature. Namely, the material used here is an electrically conductive polymer formed by blending an electrically conductive material with a polymer such as polyethylene.